Leo Bodnar’s webshop sells two GPS disciplined oscillators, and I tested the miniature version as shown in the image below connected to the airspy. It required 60dB attenuation to get a SNR on 60 dB, as a result I estimate the output of the GPS DO to be above 0 dBm when the drive current is set to 8 mA (you can increase it to 32 mA). With the rfmeter program on the VNA I verified that the output is at best 10 dBm over a 50 Ohm load.
The signal looks very clean on the waterfall plot, I could not detect spurious signals around the carrier that originate from the GPS DO. You program the frequency with the provided software for which you need a windows PC, I could not find anything that runs on a Mac or on Linux. But once it is programmed at a preset frequency the settings are retained. This makes the GPS DO ideal as a frequency reference that you can carry around, a USB charger is sufficient to keep it alive.
I verified the accuracy of the airspy SDR local oscillator which is temperature compensated (I believe), but its frequency is not disciplined to the internet or GPS. Here are the PPM offsets that I found with the sdrsharp software:
- 30 MHz: -1 ppm,
- 50 MHz: -4 ppm
- 145 MHz: -15 ppm
- 250 MHz: -13 ppm
- 430 MHz: -37 ppm
- 500 MHz: -41 ppm
- 750 MHz: -87 ppm
Very likely these values are temperature dependent, for the airspy HF+ I did a similar test for HF frequencies, in this case the calibration values were obtained by the SDR console software:
- 500 kHz : 3.40 ppm
- 1 MHz : 3.40 ppm
- 5 MHz : 2.70 ppm
- 9996 kHz : 2.65 ppm (it matches the result I got from the RWM time reference)
- 10 MHz : 2.64 ppm
- 20 MHz : 2.59 ppm
- 30 MHz : 2.58 ppm
Below 500 kHz I saw control loop oscillations in the GPS DO output, I did not attempt to estimate any ppm offset. There is an initialization time of about a second for the GPS DO to change to a new frequency, the manufacturer claims that its tuning range is between 400 Hz and 810 MHz.
Why do you need a GPS DO? If you go into the microwave domain (23cm and up) then usually any frequency comes from a lower value via frequency multiplying in which case you may want to use the GPSDO or a Rubidium or Cesium standard. For Doppler measurements to cubesats on the 2m and 70cm we want the observed frequency to be within a Hertz or better, and probably there are a dozen more applications that I forgot. Leo Bodnar’s webshop is here.
Last update: 22-jun-2018